Hereby a freebuild I've got sitting on a shelf for a looong time.
In the Northwest of the Trifork, the Twins command the only river running from the Lake of the Trifork. All the other rivers are upstream from the Lake, supplying it with melting water from the cold north, rainwater from the central plains surrounding Cedrica and muddy waters from the forests of north west Kaliphlin.
Via the water locks, the water level of the lake can be managed, including raising it to flood the lowlands south of the Trifork in case of attack or siege, limiting access to one metalled road, easing defenses.
The water lock at the Twins is one of two, allowing traffic to pass through with minimal decrease in the water levels of the lake. The second one is upstream in the middle of the city.
Apart from the towers themselves, the defenses also include a chain mechanism to block of the river (and to protect the waterlocks in times of peace).
This is a build for the HSS challenge. Thanks for looking - C&C welcome